1. Field of the Invention
The present invention concerns a laminar organic photodetector, an organic x-ray flat panel detector, method for production of a laminar organic photodetector, and a method for production of an organic x-ray flat panel detector.
2. Description of the Prior Art
With a laminar photodetector, light striking the photodetector is transduced into electrical signals that can be converted into an image data set with a suitable evaluation device. Laminar organic photodetectors, namely photodetectors with a photoactive layer made from an organic semiconductor material, represent an interesting alternative to photodetectors with a photoactive layer made from an inorganic material.
The production of an x-ray flat panel detector with an organic photodetector can be relatively cost-effective, it is the object of an x-ray flat panel detector to transduce an x-ray radiation penetrating through an examination subject (and thereby attenuated) into corresponding electrical signals that can then in turn be converted into an x-ray image data set with an evaluation device. The x-ray image associated with the x-ray image data set can be visualized with a viewing apparatus. Such an x-ray flat panel detector is known from United States Patent Application Publication No. 2003/0025084 A1, for example.
In particular, large-area photodetectors as are sometimes used for x-ray flat panel detectors, can be produced relatively cost-effectively because the organic layers of the organic photodetector can be applied over a large area with relatively cost-effective methods such as, for example, with rotation coating (spin coating), scraping or printing techniques.
FIG. 1 shows in section an example of a laminar organic photodetector PD1 for explanation of the general problem associated therewith.
The photodetector PD1 shown in section in FIG. 1 has a number of layers 1 through 7 attached to one another. The known photodetector PD1 has a laminar substrate 1 in which transistors (not shown in FIG. 1) are embedded in a matrix configuration. Each of the individual transistors is associated with one of the pixels of the image to be acquired with the photodetector PD1.
A passivation layer 2 is applied on the substrate 1, on which passivation layer 2 is structured in turn (for example by means of a lithography process) a laminar and structured electrode 3 that is shown in section in plan view in FIG. 2. For example, the structured electrode 3 is formed of gold, platinum, palladium, silver or indium-tin oxide.
As can be seen from FIG. 2, the electrode 3 is structured like a matrix and has a number of sub-electrodes 3a through 3r that are electrically insulated from one another. Each of the sub-electrodes 3a through 3r is electrically connected with one of the respective transistors of the substrate 1. Each of the sub-electrodes 3a through 3r is therefore respectively associated with one of the pixels of the image to be acquired with the photodetector PD1.
An organic hole transport layer 4 (for example made from PEDOT:PSS) is applied over the area of the laminar and structured electrode 3. A photoactive layer 5 (made from an organic semiconductor material, for example poly-3-hexylthiophene/PCBM) is in turn applied over the area of the laminar organic hole transport layer 4.
The laminar organic photoactive layer 5 connects to an unstructured, at least semi-transparent laminar electrode 6. The laminar electrode 6 is, for example, a thin metal layer made from calcium or silver. In order to protect the photodetector PD1 from damage and degradation due to oxygen and moisture, a protective layer 7 is finally applied on the electrode 6. The protective layer is formed, for example, of glass, an optimally transparent polymer, or a multi-layer system made from organic polymers and inorganic barrier layers such as Al2O3 or Si3N4.
If an image is to be acquired with the photodetector PD1, the light distribution associated with the image thus penetrates the protective layer 7 and the at least semi-transparent electrode 6 and is transduced into electrical signals by the photoactive layer 5 in connection with the hole transport layer 4 and the two electrodes 6 and 3, which electrical signals are read out with the transistors of the substrate 1. The read signals are in turn relayed to an evaluation device (not shown in FIGS. 1 and 2 but known to those skilled in the art) and are processed into an image data set. The image data set can then be visualized as an image with a viewing apparatus (likewise not shown in FIGS. 1 and 2).
The image is constructed of a number of pixels. Each of the sub-electrodes 3a through 3r of the structured laminar electrode 3 or each transistor of the substrate 1 that is connected with a corresponding sub-electrode is associated with one of these pixels.
The two organic layers 4 and 5 have a relatively high conductivity and therefore a relatively high quantum efficiency in a range from 60% to 85%. However, since the two organic layers 4 and 5 are applied unstructured over the area of on the structured electrode 3 and the two organic layers have a relatively high transverse conductivity (i.e. a conductivity parallel to their area dimensions), it leads to a relatively large crosstalk of the electrical signals destined for the respective sub-electrodes 3a through 3r or their associated transistors of the substrate 1. A limited spatial resolution of the image acquired with the photodetector PD1 is the consequence.